U.S. Pat. No. 4,361,681 teaches that polyester containing anhydride end-cap agents have a reduced acetaldehyde generation rate.
U.S. Pat. No. 5,459,224 discloses polyesters having 4-oxybenzylidene end-cap agents to impart improved weatherability and photostability, but no mention is made as to evolution of acetaldehyde. However, it is indicated that such polyesters are suitable for food and beverage packaging.
Polyesters can be synthesized by a number of routes known in the art using a variety of catalyst systems. EP 826,713 A1 teaches that lower levels of acetaldehyde occur during copolymerization of PET when a phosphite such as bis(2,4-di-tert-butylphenyl)pentaerythritol phosphite is present during the polymerization.
U.S. Pat. Nos. 4,837,115; 5,258,233; 5,266,413; 5,340,884; 5,648,032 and 5,650,469; and WO 93/20147 A1; WO 93/23474 A1; WO 98/07786 and WO 98/39388 teach the use of polyamides as a means of reducing the concentration of acetaldehyde, presumedly via a Schiff-base reaction with the aldehyde, which is reversible in the presence of water.
EP application 191,701A2 describes biaxially oriented container having excellent barrier properties said container comprising a blend of a poly(ethylene terephthalate) resin and an ethylene/vinyl alcohol copolymer resin. This reference is focused on improved gas barrier properties and is silent as to any reduction of acetaldehyde content.
Japanese Sho 62-257959 describes biaxially stretched vessels built of synthetic resin consisting of poly(ethylene terephthalate) blended with a copolymer of a polyamide, or blended with ethylene/vinyl alcohol copolymer at a weight fraction of 0.1 to 15 percent. The examples are limited to a single EVOH polymer (EVEL.RTM. G110, Kuraray Co.). It is taught that a lower level of acetaldehyde occurs when the EVOH polymer is present.
When ethylene/vinyl alcohol copolymer is added to a polyester even at a level of 0.1% by weight (1000 ppm), the resulting blend when extruded into film or plaque exhibits unacceptable haze usually seen as a graying effect. This haze is perceptible and severely limits the amount of ethylene/vinyl alcohol copolymer which can be used for the purpose of reducing the level of acetaldehyde. The instant invention is distinguished from the prior art by virtue of using lesser amounts (50-750 ppm) of ethylene/vinyl alcohol copolymer which do not produce unacceptable haze, but still provide significant reduction in the level of acetaldehyde formed during processing.
European application 714,832A1 teaches a method of manufacturing a container comprising poly(ethylene terephthalate), polycarbonate or PEN polyester with an additive in the bottle wall which binds acetaldehyde. The additive is generally described as a polyamide.
U.S. Pat. No. 5,656,221 describes a process of producing polyester with reduced acetaldehyde concentration using certain catalysts or inert gas conditions or by adding an amide compound. These include commercial polyamides or long chain aliphatic amide compounds.
U.S. Pat. No. 5,856,385 teaches the use of polyamide or amide-wax to reduce the level of acetaldehyde which occurs when sorbitol-based clarifying agent is heated in polyolefins.
The invention is useful for any polyester where aldehydic compounds, especially acetaldehyde, are formed or evolved during thermal processing of said polyester. Thermal processing of PET includes the synthesis of PET, thermal exposure during solid state polymerization (SSP), any injection molding, injection-blow molding or stretch-blow molding used in the manufacture of preforms, parisons or bottles and containers, or extrusion of film, or during any melt processing of PET above its glass transition temperature and below its decomposition temperature.
The instant invention provides for a lower amount of contaminants (e.g. aldehydes) in PET water bottles thus providing for improved taste or flavor in bottled water or other bottled beverages in said PET containers. The reduction in the amount of acetaldehyde is highly beneficial in this respect. Acetaldehyde is known as a decomposition product of polyesters such as PET. The acetaldehyde imparts an undesirable taste or flavor to bottled water stored in PET bottles. It has been a long sought objective of the industry to reduce the level of acetaldehyde which migrates out of the PET bottle walls into the water or other beverage stored therein. A number of engineering or design changes to extruders, injection molding machines for preforms and bottle making machinery have been made to minimize formation of acetaldehyde when poly(ethylene terephthalate) PET is processed. Modification to the PET composition itself have been made to lower its melting point or its melt viscosity in order to allow less severe thermal or mechanical damage when PET is processed into preforms or bottles.